Pavement construction



May 10, 1966 G. A. LEONARDS PAVEMENT CONSTRUCTION Filed March 4, 1965 in; fly 2; i 6 6 .x \Z m K M a /w OIV m 5 w n w M A W r k u w 0 0 o 0 0 Time, days 21: 2

HTTORNfY United States Patent 3,250,188 PAVEMENT CONSTRUCTION Gerald A. Leonards, West Lafayette, ImL, assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed Mar. 4, 1963, Ser. No. 262,550

Claims. (Cl. 947) This invention relates generally to highway and airfield pavements and, more particularly, relates to novel procedures for economically constructing pavements having shallow base layers providing full frost barriers when desired.

It is well known that the combination of a high water table accompanied by low temperatures, particularly as present in certain northern regions, results in What is commonly known as heaving of highway and runway surfaces. The only procedure widely used to prevent this condition is to place a grade of non-frost susceptible material, such as aggregates containing gravel and/ or coarse sands, to a depth under the pavement so that it extends below the frost or freezing line. Other solutions, such as direct heating by coils or chemical treatment, have been found inadequate. However, since non-frost susceptible materials, particularly gravel, are becoming scarce and are not particularly efiicient, there has been a long-felt desire to provide an improved highway construction.

Briefly, the present invention comprises the employment of a high insulating layer in between the layer of a nonfrost susceptible material and the frost susceptible material, which substantially reduces the depth of freezing to the point where a given depth of insulating material can besubstituted for an equivalent of several times this depth of gravel It is necessary that this insulating layer have certain qualities of insulation, heat capacity, imperviousness to vapor transport, compressibility, strength and thermal conductivity to prevent detrimental effects. The type and location of these layers are factors determined by the principles of this invention.

Accordingly, it is an object of the present invention to provide for a novel highway or airfield pavement not susceptible to the detrimental effects of freezing temperatures.

- Another object of the presentinvention is the provision of a novel pavement construction wherein an insulating layer, preferably of expanded plastic, is located between a layer of non-frost susceptible material and the frost susceptible material below the pavement.

Still another object of the present invention is toprovide a'novel pavement construction wherein the depth requirements of non-frost susceptible material are substantially reduced.

Another object of the present invention is to provide a process of highway construction whereby the thickness of an insulating layer, and the depth thereof, can be predicted for any degree-day climatic region.

A further object of the present invention is the provision of a process whereby a relatively shallow bed for a pavement construction can be provided while still rendering the pavement insusceptible to heaving even under severe climatic conditions.

Yet additional objects and advantages of the present invention, and its numerous cognate benefits and features are even more apparent and manifest in and by in the approach of the present invention, are as follows:

the ensuing description and specification taken in conjunction with the accompanying drawing in which:

FIGURE 1 is a cross-sectional view of a pavement with its base constructed according to the principles of the present invention; and

FIGURE 2 is a showing, by graphical illustration, of the relative temperatures on the'top of the insulating layer contrasted with that on the bottom of the insulating layer of FIG. 1,- under severe winter conditions.

All prior practical analytical solutions for determining the depth of frost penetration have been derived for uniform or homogeneous soils. This includes the wellknown Neumann theory, the Stephan theory and the Berggren theory. Therefore, for the non-uniform or multi-layer system which occurs typically below highway and airfield pavements, particularly when an insulating layer such as expanded plastic is used according to the present invention, only approximate computation techniques, which have proven unreliable, have heretofore been possible. A discussion of such prior solutions is typically presented in an article entitled Frost Penetration Below Highway and Airfield Pavements by Harl P. Aldrich, Jr., associate professor at the Massachusetts Institute of Technology, as published in the Highway Research Board (2101 Constitution Avenue, Washington 25, DC.) Bulletin at page 124.

All of these prior theories have made certain simplifying assumptions which seriously limit the accuracy of the frost penetration predictions, particularly when applied to layered systems. Besides the seriously limiting consideration of the soil as a semi-infinite homogeneous isothermic'material, previous theories had concluded that all the moisture in the system freezes and releases the latent heat at the temperature at which freezing was initiated. When dealing with a soil-water-air system, it has been established that the amount of frozen moisture in the system is a function of the temperature. As a result of this fact, the latent heat of fusion released at the frost line is not that of the total moisture, but is only a fraction of this, depending on the soil type. None of the works prior to the present invention consider the effects of the variation in magnitude and position of a heat source resulting from the progressive freezing of the soil water as temperatures become lowered in the soil mass.

For a homogeneous system, the assumption made by previous theories have not contributed to a very large error. However, for layeredsystems in which the layers are arbitrarily varied in position 'and/ or have wide variations in thermal properties, solutions such as the Berggren theory, have not been satisfactory.

Accordingly, prior limiting assumptions not included (1) Specific heat and thermal conductivity of the soil are independent of temperature, (2) the heat capacity of the soil can be neglected, (3) the temperature at which water first freezes in the soil pores is independent on time and space, and (4) at the freezing front, when nucleation first occurs, all of the water in the soil pores freezes instantaneously. Layered systems are treated only by crude approximations in the prior approaches.

In order to account for the variations of the above assumptions when practicing the present invention, the soil-water-air system was considered to pass through a number of arbitrary phases or temperature steps as it is cooled. Within each temperature step, the properties of specific heat, thermal conductivity, moisture content, unit Weight and fraction of water frozen, are considered. Using a sufficient number of temperature steps the actual variations in these properties can be accounted for to any desired degree. The process for which perfection of a solution for frost penetration in multi-layer systems can be achieved is dependent upon the storage capacity of computers to solve the problems presented, and is as follows:

The depth z, (t) of an isotherm at which a phase change is assumed to occur is given by where t is time and p is a constant. Making use of the well-known Fouriers differential equation and the appropriate boundary conditions describing the problem, a series of simultaneous equations containing the unknown constants p is obtained in the formula 1=P1 1 1 G(x) =Gauss error function C-=specific heat K=thermal conductivity =unit weight T=magnitude of temperature interval, and L=latent heat of fusion per unit volume, with the subscript i varying between 1 and N, the number of temperature intervals. Technique of solving the equation:

(1) Taking the above equation with i=N1, it is 'noted that Z =oo and thus G(Z )=1.0. Assuming a value of Z,, the only unknown Z,, is determined.

(2) Using the solution obtained above, the above equation with i=N-2 is solved for Z (3) The process is repeated through N-Z equations obtaining from the last step (i=2) a value of Z (4) The last equation, i=1, is used to obtain a second value of Z (5) The two values of Z are compared and if they are equal, the solutions to the equation are equal to the values Z obtained in the above steps.

(6) If the two values of Z are not equal, the process must be repeated, starting with Step 1 and assuming another value for Z To apply the above equations to a multi-layered system consisting of infinite slabs in intimate contact, the following considerations and process are used:

(1) Until cooling has penetrated through the top layer, the problem is essentially that of determining the temperature distribution in a homogeneous mass with properties of those of the top layer. The constants are determined according to the procedure given above.

(2) As each isotherm separating one phase, or temperature step, from another crosses a boundary between two layers, the properties of the layer into which that temperature step is moving is substituted into the equations and a set of new constants are determined as above.

(3) This process is repeated until the penetration of a particular isotherm has reached a specified depth.

Thus, once given a climatic or degree-day situation as well as the thermal character of the insulating material, including, of course, the factors of strength relating to the material, one can determine the exact depth and thickness of the insulating layer, and amount of base and subbase, to completely prevent frost penetration to the soil there below.

The climatic condition for any region is generally known, as for example, at Midland, Michigan an extremely severe winter will normally consist of about 2,000 degree-days, the degree-days for any one day equaling the difference between the average daily air temperature and 32 F. Without an insulating barrier, as proposed by the present invention, a fifty-two-inch thick layer of coarse gravel base under a three-inch thick asphalt pavement would appear to be necessary for a nofrost condition at 2,000 degree-days. With the high price of gravel, and its continuing scarcity, design for such a severe condition has been found impractical. As a result, it is reported that the Michigan State Highway Department designs for approximately a 1,600 to 1,750 degree-day condition, with the resultant heaving or breaking up of a portion of some highways, particularly those of asphalt, when severe winter. conditions and quick thaws are encountered.

In contrast to the above, an actual highway construction 10, developed according to the principles of the present invention and as illustrated in cross-section in FIG- URE 1, has shown that substantially greater economies can be had while providing a frost-free soil below the construction. Specifically, the highway of FIGURE 1 employs a two and one-half-inch thick substantially rigid, hard asphalt pavement 12 with a seven-inch thick base 14 formed of a non-frost susceptible aggregate, and a fourteen-inch thick subbase 16 formed of a non-frost susceptible sand, with an underlying expanded polystyrene forming a foam insulating barrier 18 of two inches in depth.

FIGURE 2 indicates that the relatively low temperatures (below freezing) penetrating the base of the highway construction to the top surface of the insulating layer 18, as depicted in FIG. 1 and over an extended winter period in Midland, Michigan, was not experienced by the bottom surface of the insulating foam layer 18. Accordingly, the frost susceptible layer below the insulating foam experienced no freezing, thereby eliminating the possibility of heaving or other deterioration due to the free-thaw conditions of the highway surface. It was also proven that the polystyrene foam, with the several inches of aggregate upon its upper surface to distribute highway loads ,across the surface thereof, has sufficient structural strength to resist substantial physical deterioration over an extended length of time. The durability of the foam, that is, its resistance to deterioration from repetitives loads and to water absorption, have proved to be entirely satisfactory. It is to be understood that other insulating layers, such as expanded urethane, might likewise be employed if economically feasible.

The approach applied to determine the structure for the degree-day situation noted for Midland, Michigan, is two-dimensional in that it considers edge losses, or that heat which can be lost from the ground on each side of the paved layer of the highway. It is because of this that the insulating layer has been extended a distance X (FIG. 1) of 13 feet beyond each edge of the asphalt layer, which itself has a width Y of 24 feet. The Midland road described above was designed for enough strength to support vehicles with as much as a 32,000 pound tandem axle load at a traffic rate of about 2,000 vehicles per day. The ability of the construction of this invention to resist repeated flexibility under such loads has been encouraging.

It is also desired that the foam insulating layer be laid so as to be substantially continuous thermally. This might be effected in several different ways. For example, the insulating material might be laid as two layers'of staggered boards, one layer substantially covering the joints of the other. The insulating material might also be laid as boards sealed together at their joints. In the aforementioned Midland road, adjacent sections of polystyrene boards were joined by a urethane plastic sealant.

Although the application of the principles of the present invention can have a wide range and scope and are applicable to any type of pavement surface, they are particularly advantageous for asphalt type pavements because the asphalt provides substantially less bridging action than concrete and is thus more reliant upon base support. While certain representative embodiments and details have been shown for the purpose of illustrating the invention, it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the invention.

Accordingly, what is claimed is:

1. A pavement construction comprising a substantially continuous surface layer adapted for vehicular traffic, a second layer of cohesionless non-frost susceptible material immediately below said surface layer, a third layer including plastic foam insulating material below said second layer, said layers resting upon a graded generally frost susceptible soil, wherein the thickness of each of said layers is determinable, relative to one another and suflicient together to provide for a frost barier from said surface layer to said soil for a given degree-day climatic region, by solving the equation .where G(x)=Gauss error function =specific heat K=thermal conductivity 'y=unit weight =magnitude of temperature interval, and L=latent heat of fusion per unit volume, with the subscript i varying between 1 and N, the number of temperature intervals. I

2. The pavement construction of claim 1 wherein said third layer comprises a polystyrene foam.

3. The pavement construction of claim 1 wherein said third layer comprises a urethane foam.

4. The pavement construction according to claim 1 wherein said surface layer comprises an asphalt material.

5. The pavement construction according to claim 1 wherein said surface layer comprises a concrete material.

6. A layered pavement construction providing a frostfree area below the base for a climatic region experiencing approximately 2,000 degree-days or less severity, comprising a thickness of pavement surfacing material, a thickness of a cohesionless non-frost susceptible material, anda thickness of approximately two inches of insulating foam having a compressive yield strength of at least 8 p.s.i. and a density no less than 0.7 lb./cu.ft., said non- 6 distribute normal highway loads over said foam without substantially crushing the latter.

8. The pavement construction of claim 7 wherein said expanded polystyrene extends laterally a distance beyond the edges of the asphalt layer of approximately one-half the lateral extent of the asphalt layer.

9. A pavement construction exposed to an alfresco en vironment experiencing freezing conditions wherein the construction provides a frost-free area below the base thereof, said construction comprising a first layer of material adapted for vehicular traffic, a second layer of cohesionless non-frost susceptable material, a third layer including a plastic foam insulating material, said third layer being substantially water impermeable and having a comfrost susceptible material and surfacing material being of pressiveyield strength of at least 8 psi. and a density no less than 0.7 lb./cu.ft., wherein said layers substantially prevent frost penetration into a frost susceptible soil therebelow and water migration upwardly therethrough.

10. A frost barrier construction comprising a layer of material adapted as a load bearing andwear resistant surface exposed to an alfresco environment experiencing freezing conditions, a layer including plastic foam insulating material located below said first-mentioned layer, said second-mentioned layer being substantially water impermeable and having a compressive yield strength of at least 8 psi. and a density no less than 0.7 lb./cu.ft., said layers having a combined thickness wherein they substantially prevent frost penetration into a frost susceptible soil therebelow and water migration upwardly therethrough.

11. The construction of claim 10 wherein said firstmentioned layer is asphalt.

12. The construction of claim 10 wherein said firstmentioned layer is concrete.

13. The construction of claim 10 wherein said plastic foam insulating material comprises substantially closed cells.

14. The frost barrier construction of claim 10 wherein said second-mentioned layer extends laterally a distance beyond the first-mentioned layer of approximately one-half the lateral extent of the first-mentioned layer.

15. A pavement construction comprising a substantially continuous surface layer adapted for vehicular traffic, a layer including plastic foam insulating material located below said surface layer, said layers resting upon a graded generally frost susceptible soil wherein the thickness of each said layer is determinable, relative to one another and sufiicient together to provide for a frost barrier from said surface layer to said soil for a given degree-day climatic region, by solving the equation with the subscript i varying between 1 and N, the number of temperature intervals.

(References on following page) References Cited by the Examiner UNITED STATES PATENTS Strahan 94-4 X Drury 94-4 Goodwin 94-9 Monroe 94-4 X Kloote.

Kloote.

8 FOREIGN PATENTS 196,435 3/1958 Austria.

5 CHARLES E. OCO NNELL, Primary Examiner.

JACOB L. NACKENOFF, Examiner.

N. C. BYERS, Assistant Examiner. 

9. A PAVEMENT CONSTRUCTION EXPOSED TO AN ALFRESCO ENVIRONMENT EXPERIENCING FREEZING CONDITIONS WHEREIN THE CONSTRUCTION PROVIDES A FROST-FREE AREA BELOW THE BASE THEREOF, SAID CONSTRUCTION COMPRISING A FIRST LAYER OF MATERIAL ADAPTED FOR VEHICULAR TRAFFIC, A SECOND LAYER OF COHESIONLESS NON-FROST SUSCEPTABLE MATERIAL, A THIRD LAYER INCLUDING A PLASTIC FOAM INSULATING MATERIAL, SAID THIRD LAYER BEING SUBSTANTIALLY WATER IMPERMEABLE AND HAVING A COMPRESSIVE YIELD STRENGTH OF AT LEAST 8 P.S.I. AND A DENSITY NO LESS THAN 0.7 LB./CU.FT., WHEREIN SAID LAYERS SUBSTANTIALLY PREVENT FROST PENETRATION INTO FROST SUSCEPTIBLE SOIL THEREBELOW AND WATER MIGRATION UPWARDLY THERETHROUGH. 